Germ cells are unipotent stem cells that divide to produce gametes in sexually reproducing organisms. A germ cell undergoes meiotic cell division to produce genetically unique, haploid sex cells, which then fuse into a diploid zygote during fertilization. In female organisms, germ cells form egg cells and in men they produce sperm cells.
What is a germ cell?
Germ cells are the cells that produce germ cells in all sexually reproducing organisms. In vertebrates, they are the precursors of male sperm cells and female egg cells. Taken together, all the germ cells of an organism are called Germline.
The germline is from the somatic (AKA body) cell line in the early stages of embryonic development. The two cell lines have very different functions; Somatic cells make up all the structures in the body, while germ cells produce gametes to pass on genetic information to offspring. The key characteristic of germ cells is that they divide through Meiosis, in contrast to all other cells in the body that divide mitotically.
Are germ cells unipotent Stem cells, as they can only become a different type of cell (an egg or a sperm). However, once fused together, they form a zygote With totipotent Potential. (ie it can lead to all other types of body cells).
Function of germ cells
Germ cells are the type of stem cells that make gametes. They are therefore the cells of origin of all sexually reproducing organisms and enable individual conspecifics to pass on genetic information to their offspring. Inheritance of DNA is the driving force behind natural selection and evolution, and the fact that germ cells divide by meiosis ensures maximum genetic variation among gametes.
Development of germ cells
In humans, germ cells come from stem cells called Primordial Germ Cells (PGCs) in a region of the embryo called epiblast. The PGCs migrate into the developing gonads, where they divide through meiosis and become either sperm or egg cells.
Meiosis does not produce identical, haploid Cells that each contain a single set of chromosomes. During fertilization, haploid egg and sperm cells fuse into a zygote that contains DNA from both parents. This genetic information remains in the germ line of the new organism and is passed on through further generations of reproduction.
Germ cells and meiotic division
Germ cells are the only cells in the body that divide Meiosis, in contrast to mitosis. While mitosis produces two diploid, genetically identical daughter cells, meiosis produces four haploid, genetically unique daughter cells.
Germ cells are diploid cells that divide into gametes, which must be haploid to ensure that the offspring get the correct number of chromosomes. Therefore, the chromosome number of the germ cells is halved by meiosis. The fact that meiosis produces genetically unique cells is also an important aspect of gamete production as it allows for greater genetic diversity within the species.
Stages of meiosis in germ cells
Meiosis takes place in two stages; Meiosis I and Meiosis II. DNA replication takes place in the interphase of the cell cycle before meiosis I begins. The germ cell divides twice during a complete round of meiosis, giving rise to four haploid gametes.
When a germ cell enters prophase I, it has already made copies of all of its DNA. At this point in time, each chromosome in the germ cell consists of two genetically identical sister chromatids, which are identified by a Centromere.
During prophase I the meiotic spindle formed and the chromosomes condense and shape homologous couples. The homologous pairs of chromosomes swap in a process called cross. This ensures that the sister chromatids are no longer genetically identical.
The homologous pairs line up in the middle of the germ cell, and spindle fibers attach to one chromosome in each pair.
The meiotic spindle fibers pull on the chromosomes, pulling one of each pair to the opposite poles of the cell. At this point in time, the sister chromatids are still bound to one another.
Telophase I and cytokinesis
The two sets of chromosomes collect at either end of the cell. The cell’s cytoplasm divides into two parts, creating two non-identical daughter cells (this is Cytokinesis). The daughter cells produced by meiosis I are haploid because they contain only half as many chromosomes as the original germ cell.
The two haploid daughter cells of meiosis I undergo another cell division in meiosis II. During prophase II, the chromosomes condense again and the meiotic spindle re-forms.
The chromosomes line up in the middle of the cell. Spindle fibers attach to each of the sister chromatids.
The spindle fibers pull the sister chromatids apart, and one from each chromosome travels to each end of the cell.
Telophase II and cytokinesis
The two cells divide to produce four genetically unique cells, each with a single set of chromosomes. In male organisms, meiosis produces four sperm cells and is called Spermatogenesis. In women, meiosis produces a single egg cell and four Polar body (small, haploid cells that cannot be fertilized) and is called Oogenesis.
Location of the germ cells
Germ cells are found in the Gonads (or genital organs) or an organism. In male vertebrates they are located in the testicles and in female vertebrates in the ovaries.